a) Field of the Invention
The invention is directed to a standard for wavelength and intensity for spectrometers, particularly for the physical and chemical analysis of substances and for determining the characteristics of these substances and for process control, in particular in the chemical industry. This standard is preferably provided for calibrating and testing measurement heads for spectrometers which are used in the near infrared range (NIR) of the spectrum and in transmission measurements and measurements in reflected light.
b) Description of the Related Art
The disclosure of European Pharmacopoeia 5.0, “2.2.40. Near-infrared spectrophotometry”, pages 59 to 61, includes arrangements for calibrating spectrometers for measuring substances and samples with radiation in the near infrared region of the spectrum with wavelengths between 780 nm and 2500 nm. Qualitative and quantitative physical and chemical information about the substances and samples being examined can be obtained in this way. However, there is no direct comparison of the spectrum of a substance that is tested with a reference spectrum of a chemical reference substance as is conventional in IR absorption spectroscopy.
It is well known in infrared spectroscopy to apply the following measurement methods: transmission measurements, measurements with diffuse reflection, and a combined method in which a diffusely reflecting reflector is arranged behind the substance or sample to be tested, e.g., liquids and solids which are transparent for infrared (IR) radiation of the wavelengths in question.
Measurements with diffuse reflection are generally applied in solids. In this connection, it must be ensured that the measurement conditions are as reproducible as possible from one sample to the next. The reflected radiation of a reflecting background reference (standard) is scanned (tested) in order to obtain a wavelength standard as a reference base for the sample measurements. Materials used for the standard include ceramic materials, perfluoropolymers, methyl chlorides, or gold. It is disadvantageous that only spectra possessing the same optical properties can be measured and compared to standards. Further, the particle size, water absorption and state of solubility must be taken into account.
In the combined method mentioned above, a reflector is arranged behind the sample to increase the optical path length. The sample is tested in a cell with a suitable diffusely reflecting reflector comprising either a suitable metal or an inert material such as, e.g., titanium dioxide (TiO2) which does not absorb radiation in the infrared spectral region in question and retains its properties without being influenced by environmental conditions such as, e.g., humidity, over a long period of time.